1. Field of the Invention
This invention relates to a semiconductor device, and more particularly relates to an electric power semiconductor device having a high withstand voltage structure.
2. Description of the Related Art
An electric power semiconductor device having a super-junction structure and including a vertical power MOSFET with a stripe cell structure is in wide use. Referring to FIG. 7 of the accompanying drawings, p type impurities are diffused via bottom surfaces and side surfaces of deep trenches 103 formed in an n type epitaxial layer 102, which is on an n type silicon substrate 101, thereby obtaining p type semiconductor regions 104 in the shape of a long column. The deep trench 103 is filled with an insulant 110. A RESURF (reduced surface field) structure is thus accomplished.
The vertical power MOSFET is fitted between the p type semiconductor regions 104, and has an n type drain region, a p type base region 105, an n type source region 106, a gate insulating film 107, and a gate electrode 108. The n type drain region is constituted by the n type epitaxial layer 102, and the n type silicon substrate 101. A drain electrode 111 is electrically connected to a rear surface of the n type silicon substrate 101 while a source electrode 112 is electrically connected to the n type source region 106 on a front surface of the n type silicon substrate 101.
With the foregoing electric power semiconductor device, when a voltage is applied across the drain electrode 111 and the source electrode 112, depletion layers spread from a pn junction on an interface between the p type semiconductor regions 104 and the n type epitaxial layer 102. When the applied voltage becomes equal to a given value, the depletion layer spreading from one p type semiconductor region 104 and the n type epitaxial layer 102 and the depletion layer spreading from another p type semiconductor region 104 and the n type epitaxial layer 102 are united each other. Therefore, the united depletion layer extends substantially over the n type epitaxial layer 102 sandwiched between the p type semiconductor regions 104, which weakens an electric field, and enables the electric power semiconductor device to have a high withstand voltage. However, at an area outside the RESURF structure which is at right and left parts shown in FIG. 7, neither the electric field is weakened nor the depletion layer expands sufficiently. Therefore, it is very difficult to accomplish the high withstand voltage.
As shown in FIG. 8, each deep trench 103 has a plurality of planar stripes on an element area at the central area of the n type epitaxial layer 102. The planar stripes extend in a Y-direction (from a third side 203 to a first side 202), and in an X-direction (from a fourth side 204 to a second side 202). The p type semiconductor regions 104 are made by diffusion onto the n type epitaxial layer 102 and along the deep trenches 103, and is in the shape of planar stripes. In other words, the deep trenches 103 and the p type semiconductor regions 104 have the RESURF structure in which the depletion layer extends in the Y-direction.
Further, with the power semiconductor device shown in FIG. 8, a high withstand voltage section exists at an element end area around the second side 202 and the fourth side 204 on a main surface of the n type epitaxial layer 102. The high withstand voltage section is in the shape of planar stripes extending in the X-direction, and is constituted by a plurality of trenches 103R arranged in the Y-direction, p type semiconductor regions 104R diffused on the n type epitaxial layer 102 and extending along the trenches 103R, and an insulant 110R filled in the trenches 103R. In short, the high withstand voltage section has the RESURF structure extending in the X-direction. When the applied voltage becomes equal to the given value and the depletion layer spreads on the n type epitaxial layer 102 and the p type semiconductor regions 104R, the high withstand voltage section reduces a difference between a total amount of positive charges on the n type epitaxial layer and a total amount of negative charges on the p type semiconductor regions 104R, balances the electric charges, and weakens the electric field.
The high withstand voltage section enables the depletion layer to efficiently and quickly spread not only in the Y-direction but also in the X-direction. Therefore, the high withstand voltage section can suppress concentration of the electric field and improve the withstand voltage (element withstand voltage) of the electric power semiconductor device.
It is assumed that the electric power semiconductor device includes the high withstand voltage section which has the RESURF structure not extending in the X-direction but extending only in the Y-direction. In such a case, when the applied voltage becomes equal to the given value, the depletion layer is prevented from extending in the X-direction by the insulant 110. In order to change adjacent RESURF layers into a depletion layer, it is necessary to make a route for discharging electron-hole from the RESURF layers.
For instance, such the route for discharging electron-hole can be secured by providing a wiring for electrically connecting the p type semiconductor regions 104 via the insulant 110 filled in the deep trenches 103. However, since the wiring enlarges the element end area, it is very difficult to improve an integration degree of the electric power semiconductor device. The high withstand voltage section shown in FIG. 8 is effective in extending the depletion layer, and improving the high withstand voltage of the electric power semiconductor device. One example of the foregoing electric power semiconductor device is disclosed in Japanese Patent Laid-Open Publication No. 2003-086800.
The electric power semiconductor device of FIG. 8 seems to suffer from the following problems: the depletion layer shown in FIG. 9 does not uniformly spread at a corner of the first side 201 and the fourth side 204, i.e., at a corner of the element end region, and concentration of the electric field and lowering of the withstand voltage are caused. Further, the foregoing problems have been observed all of four corners of the first to fourth sides 201 to 204.